1. Field of the Invention
The present invention relates to a liquid ejection head for ejecting liquid droplets such as ink droplets and also to a substrate therefor. More particularly, the present invention relates to a liquid ejection head for ejecting liquid droplets by means of thermal energy.
2. Description of the Related Art
Techniques of electrically energizing a heat generating resistor to make it generate heat, bubbling ink by means of the heat generated by the heat generating resistor and causing ink droplets to be ejected from an ejection port under bubble pressure for recording purposes are known. With such a technique, the thermal energy generated for the purpose of ejecting ink is partly accumulated with time in the liquid ejection head that includes a base body on which heat generating resistors are mounted so that the temperature of the liquid ejection head gradually rises. Then, as a result, the temperature of the ink to be ejected from the liquid ejection head rises to by turn reduce the viscosity of the ink. The net result will be an increase in the quantity of ink droplets ejected from an ejection port of the liquid ejection head per unit time that by turn gives rise to an uneven density on the part of the image printed by the ejected ink.
Liquid ejection heads of the same type represent dispersion in terms of the resistance values of the wiring, the heat generating members and the driving devices of liquid ejection heads. As means for absorbing such dispersion, the liquid ejection head is designed to apply energy to the heat generating resistors thereof by about 1.2 times of the minimum electric power (or the minimum voltage) required for the liquid ejection head to bubble ink. This is one of the hidden reasons for producing such dispersion.
Under the above-described drive conditions, the surface temperature of the heat generating section of the liquid ejection head keeps on rising after bubbling ink in the above-described manner due to the excessive energy applied to that section. Then, as a result, the thermal stress in the liquid ejection head increases to give rise to a problem of an undesirably limited service life of the liquid ejection head.
Therefore, application of excessive energy is not desirable for liquid ejection heads of the above-described type. In view of this problem, Japanese Patent Application Laid-Open No. 2005-231175 proposes arranging a temperature sensor or a bubbling detection sensor on the surface of the heat generating section.
However, with the technique of detecting the surface temperature of the heat generating section of a liquid ejection head, there is no knowing if the surface of the heat generating section that contacts ink is in a state of nuclear boiling or in a state of film boiling. Then, there arises a difficulty of rectifying the rate at which bubbling energy is applied.
On the other hand, with the technique of detecting bubbling by arranging two electrodes in a region on the heat generating section of a liquid ejection head, ink on the heat generating section conducts electricity between the two electrodes and the application of a drive signal to a heat generating resistor is blocked when ink no longer exists between the electrodes due to growth of bubbles on the heat generating section. While bubbles need to spread over an area that is necessary for ink ejection (necessary bubbling region), with the technique of arranging two electrodes on the heat generating section, however, the positions and the sizes of the electrodes are subjected to limitations that are determined as a function of the necessary bubbling region. Therefore, one of the electrodes may be covered with a bubble before the bubble grows to a size that allows it to apply ejection energy to ink to prematurely block the application of a drive signal to the heat generating resistor, depending on the manner of spreading of bubbles. Then, as a result, the quantity of ejected liquid droplets or the rate of ejection of liquid droplets become instable to consequently degrade the quality of the image printed by the ejected ink.